Optical modulator leaps size, power bars

Portland, Ore.  Electrical engineers at the University of Texas (Austin) have demonstrated what they claim is the world's smallest silicon modulator. The device features a photonic-crystal waveguide with an electrode configuration that they hope will make it easily manufacturable. Such a compact modulator might be the key to building practical all-silicon lasers, they said.

The device was presented at the IEEE's 4th International Conference on Optical Communications and Networks in Bangkok, Thailand.

Photonic crystals are periodic structures in silicon. They are usually defined by a regular set of holes that interact with photons in the same way that the much smaller periodic structure of silicon atoms interacts with electrons. The structures can produce superior optical cavities when optical bandgaps are introduced into silicon.

"Our main contribution is the size reduction and reduction of power consumption, but our electrode-routing scheme makes us unique," said Wei Jiang, a researcher on the project.

Jiang described the routing scheme the group used. "We routed the central electrode to an outside pad. In this way, there is no need to place a rather big pad between the two arms" of the Mach-Zehnder structure. Therefore, our structure is slimmer. For any realistic modulator device, the central electrode must be routed to the driving circuitry for an outer pad. So our device is much closer to a real modulator."

The architecture realizes a Mach-Zehnder interferometer (MZI), which modulates by comparing the phase at the end of two arms of a signal path. One is unobstructed and the second is electrically controlled by the photonic-crystal waveguide, thereby enabling an electrical control signal to modulate the optical signal.

To make the design the world's smallest, and thus manufacturable, the two electrical connections (one on each side of the 80-micron-long photonic crystal in one arm of the MZI) had to go through an extra step that created an external connection to the central electrode.

"Others have demonstrated silicon modulators, but they either used conventional rib guides instead of photonic crystals, or they did not make their design manufacturable. From the beginning, we knew that a manufacturable device requires an electrode in the middle between its two arms," said Jiang.

"Our device can be used as an external modulator for future electrically pumped lasers on silicon chips for analog signal transmission," said EE Ray Chen, a professor at the University of Texas.

The ultracompact silicon electro-optical modulator works by slowing light down in one of the two arms of a Mach-Zehnder interferometer, thereby shifting its phase to control the frequency of modulation of the optical signal passing through the photonic crystal. The electrical signal injected just .15 milliampere into one arm of the dual photonic crystals to achieve a 92 percent modulation depth in optical communication wavelengths of 1.3 to 1.55 microns.

"Basically, to modulate the light inside the waveguide, we need to slow down the coherent light by a half-wavelength, which is achieved by external current injection. The faster you can slow down the light, the smaller the amount of current and therefore power is needed to modulate the light within. Since the photon propagates much more slowly under current injection, the device size can be considerably smaller than [it is in] conventional devices," said Chen.

Silicon is transparent in the optical-communications wavelengths and has a high index of refraction, enabling light to pass through chips fabricated on a standard CMOS line. The photonic crystal enhances the ability to slow the propagation of light through the waveguide under the control of an electrical current.

The group fabricated its photonic-crystal modulator atop a silicon-on-insulator wafer with a 215-nanometer silicon core and buried oxide layer of 2 microns. The pitch of the hexagonal lattice for the holes was 400 nm.

While others have demonstrated silicon optical modulators using conventional rib waveguides, the group's modulator is up to 100 times smaller.

"The hole size can be as small as 100 nm and photons can be slowed down 100 times compared to conventional index-guiding mechanisms, therefore significantly reducing driving power and device size," said Chen.

In addition to a reduction in size, the researchers report that the much shorter electrodes enable their photonic-crystal waveguide to consume up to 100-fold less power when compared with conventional rib waveguides.

To clear the remaining engineering hurdles to silicon photonics, the researchers plan first to optimize their current design, then use it to electrically pump a silicon laser, thereby heralding the beginning of a new regime of silicon technology.

As they optimize the design, the researchers aim to "reduce coupling loss and demonstrate 10-Gbit-per-second modulation speed," said Chen.

"Then," said Jiang, "we can use the active electrical control of our modulator to control silicon lasers. But first we need a better understanding of the photon-electron interaction that makes lasing by injecting electrons. Also, from the fabrication point of view, we need to master electron-beam lithography and multilayer alignment, which will be very complicated, since we may need about seven layers. So a lot of alignment and other challenges will need to be overcome at this small scale. But for now, we think we are ahead of everyone else."

The researchers also hope to improve the device's performance by further optimizing the buried electrode and reducing contact resistance.

Funding for the project was provided by the Air Force Office of Scientific Research and Omega Optics Inc. (eomegaoptics.com).